To calculate the water temperature in step 3 of a water-cooled compressor system, key information needed includes the flow rate of glycol and seawater, the material being compressed (air), and its inlet and outlet temperatures. The discussion mentions that the water temperature in step 2 can be assumed between 288 K and 313 K, with a specific suggestion of 300 K. A 300 kW compressor typically requires about 20 kW of cooling, which can be derived from manufacturer data. The flow rate of the glycol solution is noted as 6.8 liters per second, and the specific heat of the glycol solution is necessary for calculating the temperature rise. Accurate calculations depend on having complete data, including the air flow rate and specific heat values.